1. Technical Field of the Invention
The present invention relates in general to a sonic well logging tool for radially evaluating cementing conditions about casing in cased wells, and in particular to a method and apparatus for circumferentially evaluating the placement and bonding strength of cement sealing material disposed about the exterior surface of a tubular member within a wellbore.
2. Description of the Prior Art
Prior art sonic well logging tools have been used for evaluating cement bonding about casing within wellbores. Typically, cement bonding is a term which has been used to describe a measure of an average compressive strength of cement disposed about a section of casing, which provides an indication of cement conditions within the wellbore, such as proper cure, mixture with borehole fluids, and voids or channeling within a cement sheath. In general, cement bonding measurements are used to provide an indication of cement placement about a well casing to determine whether the cement provides an adequate fluid seal to prevent fluids from flowing between portions of a wellbore.
Prior art sonic well logging tools have been used for radially determining cement conditions within cased wellbores, from which the circumferential placement of cement about the exterior of a casing can be evaluated. For example, pulse-echo type of sonic well logging tools have been used to transmit an initial sonic pulse radially outward from a sonic transducer to a spot on a casing wall, and then listen for a reflected sonic signal, or echo, with the same sonic transducer used for transmitting the initial sonic pulse. Pulse-echo sonic well logging tools typically operate at high ultrasonic frequencies ranging from 200 kHz to 500 kHz.
One example of a pulse-echo sonic tool is disclosed in U.S. Pat. No. 3,369,626, invented by J. Zemanek, Jr., entitled "Methods of an Apparatus for Production of a Visual Record of Physical Conditions of Materials Transversed by a Borehole", and issued Feb. 20, 1968. Another example is shown in U.S. Pat. No. 4,255,798, invented by Havira, entitled "Method and Apparatus for Acoustically Investigating a Casing in Cement Bond in a Borehole", and issued Mar. 10, 1981. Yet another example is set forth in U.S. Pat. No. 4,709,357, invented by Maki, Jr., entitled "Method and Apparatus for Acoustically Investigating a Borehole Casing Cement Bond", and issued Nov. 24, 1987. U.S. Pat. Nos. 3,369,626, 4,255,798, and 4,709,357 are hereby incorporated by reference as if fully set forth herein.
Prior art pulse-echo tools present several problems in radially determining cement conditions within cased wellbores. Prior art pulse-echo tool readings are affected by the presence of oil, gas, and muds having a heavy weight. Further, pulse-echo tools may not detect thin cement sheaths, such as cement sheaths having a radial wall thickness which measures less than one-half inch. Additionally, prior art pulse echo tool transducers have a spot measurement size of about one inch which may not detect smaller channels in cement, such as a channel which extends less than twenty-five percent about the circumference of a seven inch casing.
Another prior art sonic well logging tool for use in radially determining cement conditions within cased wellbores has six radially extending, motorized arms for radially pressing twelve transducers against the interior surface of casing, as disclosed in a Western Atlas International sales brochure, entitled "Cement Evaluation", and dated 1990. This prior art sonic well logging tool passes the sonic signal directly to the casing, rather than passing the sonic signal through a borehole fluid to reach the casing wall.
This later type of sonic well logging tool presents several problems in determining radial cement bonding conditions. One problem is that good surface contact must be maintained between the casing and the transducers for the tool to work properly. If good surface contact is not maintained with the casing, the sonic signal may be attenuated in areas of poor cement bonding, resulting in data readings which indicate areas of good cement bonding where the cement bonding is actually poor. Additionally, the radially extending, motorized arms only operate properly over a narrow range of casing sizes, as opposed to the type of sonic well logging tools which transmit a sonic signal through a borehole fluid.
Prior art sonic transducers for radially determining cement conditions typically transmit ultrasonic signals having frequencies which range from 200-500 kHz , and which attenuate rapidly after traveling only a very short distance along a well casing. This type of sonic transducer is not suitable for passing a sonic signal through both a borehole fluid and longitudinally along the casing wall. Additionally, prior art sonic transmitters for radially determining cement conditions are typically made from lead-metaniobate, which transmits a sonic signal over a wide spectrum of frequencies at low amplitudes. That is, prior art sonic transmitters typically transmit sonic pulses which are spread over a broad frequency range to provide a low energy in a broad range of frequencies. However, for radially measuring cement conditions longitudinally along casing wall, it is desirable to provide sonic transducers which transmit high energy levels at few frequencies, such that the sonic signal will not attenuate rapidly.
Another problem with prior art sonic well logging tools for radially determining cement conditions is that data readings from the downhole tool are often digitally transmitted, requiring additional downhole equipment which is both expensive and subject to failure. Additionally, costs are typically greater for uphole equipment used to receive and process data from a digital transmission, as opposed to processing data which is transmitted in an analog signal.
Additionally, prior art sonic well logging tools for radially determining cement conditions circumferentially about a well casing are operated on multiconductor wireline cables. Often, it is desirable to operate sonic well logging tools on monocables, that is, wireline cables having only a single insulated conductor. However, prior art sonic well logging tools for radially determining cement conditions about the circumference of a well casing cannot be operated on monocable wirelines, requiring multiple conductors for operation within a tool string.